1. Field of the Invention
The present invention relates to a piezoelectric element and a mobile communication device using the same.
2. Description of the Prior Art
The recent trends of speeding up information processing terminals and digitalization of communication require compact and high frequency vibrators and compact and wide band intermediate frequency filters. For these piezoelectric elements, energy trapping piezoelectric elements conventionally have been used, but the recent trends require more compact and wider band piezoelectric elements.
Crystal filters are used most widely as the energy trapping piezoelectric filters. However, it is difficult to form a wide band filter with quartz, because the material has a small electromechanical coupling factor.
For this reason, piezoelectric single crystals or piezoelectric ceramics that have a large electromechanical coupling factor are used for these applications. Among the piezoelectric single crystals, an X-cut LiTaO3 single crystal, which has a high Q of vibration and excellent temperature characteristics, is used most commonly.
LiTaO3 single crystal has large piezoelectricity and is suitable for a wide band filter, but undesired vibration tends to be excited so that large spurious (spurious response) is likely to occur. Therefore, thickness shear vibration having relatively small spurious is used as the vibration mode. The thickness shear vibration refers to vibration whose displacement direction is substantially identical to its propagation direction. Thickness twist vibration whose propagation direction is perpendicular to its displacement direction also is excited in the LiTaO3 single crystals, which also causes spurious. Furthermore, reflections of these vibrations at the end faces of oscillating elements also cause spurious.
In order to reduce the influence of the reflections at the end faces of the thickness shear vibration and the thickness twist vibration for further compact piezoelectric vibrators, a method for forming a piezoelectric vibrator in a strip has been known.
When using a X plate of LiTaO3 single crystal as a substrate of a strip vibrator, a substrate that is cut in a direction of xe2x88x9250xc2x0xc2x12xc2x0 from the Y-axis has the lowest resonance impedance, and therefore usually this direction is used as the displacement direction of vibration. FIG. 8 is a plan view of an example of such a conventional LiTaO3 piezoelectric vibrator.
Referring to FIG. 8, a conventional piezoelectric vibrator 1 includes a piezoelectric substrate 2 formed of LiTaO3 and a pair of exciting electrodes 3 formed on one principal plane and another principal plane of the piezoelectric substrate 2. In FIG. 8, an electrode for interconnection is not shown. As shown in FIG. 8, in the conventional LiTaO3 resonator in general, the piezoelectric substrate is cut in such a manner that the displacement direction of the thickness shear vibration corresponds to the longitudinal direction, and the exciting electrodes 3 are formed along the entire width in the traverse direction of the piezoelectric substrate 2.
In this vibrator, it is necessary to cut a substrate into a strip with a small width (about several times the thickness) in order to avoid undesired vibration in connection with the width. Therefore, when the direction in which the substrate is cut is significantly different from the direction of the cleavage of the crystal, the element is broken readily in the cleavage direction when cutting the substrate. Thus, miniaturization was difficult. Moreover, if chipping occurs in a cut plane, the chipping on the cut plane causes spurious. Thus, it was difficult to form a high precision vibrator or filter that operates at high frequencies.
As an attempt to solve this problem, a piezoelectric vibrator employing a piezoelectric substrate cut in the direction xe2x88x9257xc2x0 from the Y-axis was proposed (see JP 6-303090A). The cleavage plane of the X plate of LiTaO3 single crystal is in the direction xe2x88x9257xc2x0 from the Y-axis, and therefore cutting along this cleavage plane can improve the processability.
However, there is still a problem in that the spurious caused by the shape of the element is likely to occur even if the piezoelectric substrate cut in the direction xe2x88x9257xc2x0 from the Y-axis is used.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a piezoelectric element that can suppress undesired spurious and allows operations at high frequencies, and to provide a mobile communication device using the same.
A piezoelectric element of the present invention includes a piezoelectric substrate, and a pair of electrodes formed on at least one principal plane selected from the group consisting of a first principal plane of the piezoelectric substrate and a second principal plane opposed to the first principal plane, wherein thickness shear vibration occurs, and the vibration direction of the thickness shear vibration is nonparallel to a side wall of the piezoelectric substrate. In the piezoelectric element of the present invention, undesired vibration such as thickness twist vibration can be suppressed, substantially without affecting the thickness shear vibration. Therefore, according to the above-described piezoelectric element, undesired spurious can be suppressed.
In the above-described piezoelectric element, it is preferable that the piezoelectric substrate has a shape of an elongated rectangular solid, and each of the pair of electrodes is formed along the entire width in the traverse direction on the at least one principal plane. This embodiment provides a strip-type piezoelectric element that allows miniaturization. In this specification, the traverse direction refers to a direction perpendicular to the longitudinal direction of the elongated rectangular solid-shaped substrate.
In the above-described piezoelectric element, it is preferable that the piezoelectric substrate is formed of LiTaO3 single crystal, the pair of electrodes includes a first exciting electrode and a second exciting electrode, the first exciting electrode is formed on the first principal plane, the second exciting electrode is formed on the second principal plane, the side wall is a side wall in the longitudinal direction of the piezoelectric substrate, and the normal line of an edge of the first exciting electrode and the normal line of an edge of the second exciting electrode are parallel to each other and are nonparallel to the side wall in the longitudinal direction. This embodiment provides a piezoelectric vibrator that can be produced easily, has particularly small spurious, and allows operations at high frequencies. In this specification, the side wall in the longitudinal direction refers to a principal plane adjacent to the first principal plane in the traverse direction
In the above-described piezoelectric element, it is preferable that the first and second principal planes are X planes of the piezoelectric substrate, and the side wall in the longitudinal direction is parallel to a plane obtained by rotating the XY plane of the piezoelectric substrate for 56xc2x0 to 58xc2x0 clockwise about the X-axis. This embodiment provides a piezoelectric vibrator that can be produced particularly easily, has even smaller spurious, and allows operations at high frequencies. In this specification, when xe2x80x9cclockwise rotation about the X-axisxe2x80x9d is referred to, the substrate is viewed from the plus direction of the X-axis (see FIG. 1A). The term xe2x80x9cX plane of the substratexe2x80x9d as used in this specification includes substrates cut at planes that are inclined several degrees from the X plane of the substrate as equivalents.
In the above-described piezoelectric element, it is preferable that the direction of the normal line is a direction obtained by rotating the Y-axis of the piezoelectric substrate for 47xc2x0 to 56xc2x0 clockwise on the YZ plane. This embodiment provides a piezoelectric vibrator that has even smaller spurious.
In the above-described piezoelectric element, it is preferable that the width W and the thickness H of the piezoelectric substrate satisfy the relationship 2.9xe2x89xa6W/Hxe2x89xa63.1 or 3.7xe2x89xa6W/Hxe2x89xa63.9. This embodiment provides a piezoelectric vibrator with good characteristics in which the spurious that occurs in connection with the width is sufficiently apart from the frequency band of the principal vibration. In the conventional element that is cut along the cleavage direction of the substrate, spurious would occur within the frequency band of the principal vibration if the ratio of the thickness and the width is set to be in a small range from 1.5 to 2.0, and thus good characteristic cannot be obtained (see Japanese Patent NO. 2855208). On the other hand, the piezoelectric vibrator of the present invention can be provided with good characteristics even if the vibrator has a large width for easy processing. In this specification, xe2x80x9cthe width of the substratexe2x80x9d refers to the distance between two side walls that extend in the longitudinal direction, and xe2x80x9cthe thickness of the substratexe2x80x9d refers to the distance between the first principal plane and the second principal plane.
In the above-described piezoelectric element, the piezoelectric substrate may include a first piezoelectric substrate and a second piezoelectric substrate, and the first piezoelectric substrate and the second piezoelectric substrate may be stacked in such a manner that their polarization directions are opposite to each other. This embodiment provides a piezoelectric vibrator that can excite thickness shear vibration of second harmonics, which generally cannot be excited, and can be operated at even higher frequencies.
In the above-described piezoelectric element, it is preferable that the side wall in the longitudinal direction is a plane cleaved by laser irradiation. This embodiment prevents chipping and an increase of surface roughness on the cleavage plane, and thus provides a piezoelectric vibrator with good element characteristics.
In the above-described piezoelectric element, it is preferable that the piezoelectric substrate is formed of LiTaO3 single crystal, the pair of electrodes includes an input electrode and an output electrode, the input electrode and the output electrode are formed on the first principal plane of the piezoelectric substrate, the piezoelectric element further comprises a ground electrode formed on the second principal plane, the side wall is a side wall in the longitudinal direction of the piezoelectric substrate, and the normal line of an edge of the input electrode and the normal line of an edge of the output electrode are parallel to each other and are nonparallel to the side wall in the longitudinal direction. This embodiment provides a piezoelectric filter that can be produced easily, has particularly small spurious, and allows operations at high frequencies.
In the above-described piezoelectric element, it is preferable that the first and second principal planes are X planes of the piezoelectric substrate, and the side wall of the piezoelectric substrate is parallel to a plane obtained by rotating the XY plane of the piezoelectric substrate for 56xc2x0 to 58xc2x0 clockwise about the X-axis. This embodiment provides a piezoelectric filter that can be produced easily, has even smaller spurious, and allows operations at high frequencies. In the above-described piezoelectric element, it is preferable that the direction of the normal line is a direction obtained by rotating the Y-axis of the piezoelectric substrate for 47xc2x0 to 56xc2x0 clockwise on the YZ plane. This embodiment provides a piezoelectric filter that has even smaller spurious.
In the above-described piezoelectric element, it is preferable that the width W and the thickness H of the piezoelectric substrate satisfy the relationship 2.9xe2x89xa6W/Hxe2x89xa63.1 or 3.7xe2x89xa6W/Hxe2x89xa63.9. This embodiment provides a piezoelectric filter with good characteristics.
In the above-described piezoelectric element, the piezoelectric substrate may include a first piezoelectric substrate and a second piezoelectric substrate, and the first piezoelectric substrate and the second piezoelectric substrate may be stacked in such a manner that their polarization directions are opposite to each other. This embodiment provides a piezoelectric filter that can be operated at even higher frequencies.
In the above-described piezoelectric element, it is preferable that the side wall in the longitudinal direction is a plane cleaved by laser irradiation. This embodiment prevents chipping and an increase of surface roughness on the cleavage plane, and thus provides a piezoelectric filter with good element characteristics.
A mobile communication device of the present invention includes the piezoelectric vibrator of the present invention as described above. According to the mobile communication device including the piezoelectric element of the present invention, a mobile communication device with stable reference frequency and stable operation can be obtained. The piezoelectric vibrator of the present invention hardly is affected by frequency hopping due to the spurious, and the amount of attenuation of signals outside the frequency band is large, and the insertion loss is small. Therefore, according to the mobile communication device including the piezoelectric filter of the present invention, a mobile communication device with high selectivity of signals can be obtained.
As described above, the piezoelectric element of the present invention can provide a piezoelectric device that has even less spurious than a conventional piezoelectric element and allows operations at high frequencies. The present invention can provide a piezoelectric element suitable for a mobile communication device or information equipment.
Furthermore, the mobile communication device using the piezoelectric vibrator of the present invention can provide a compact and high-speed mobile communication device with high operation frequency.
The piezoelectric filter of the present invention hardly is affected by frequency hopping due to spurious, and the amount of attenuation of signals outside the frequency band is large, and the insertion loss is small. Therefore, according to the mobile communication device including the piezoelectric filter of the present invention, a mobile communication device with high selectivity of signals can be obtained.